1. Analyze the caller IP phone logs, in the failed calls , the caller side sent out the invite message to U1980, after 11 seconds didn’t receive 180 message, then the caller side sent a Cancel message to release the call.
2. Analysis U1980 logs, U1980 have received the Invite message from Caller side and sent the invite message to called side, but without any response, then U1980 keep sending invite message to called user.
3. Analysis called side IP phone logs, the called user didn't receive U1980 invite message, confirm there is a network issue between U1980 and called side.
4. After customer fixed the network, the no ring back issue didn’t occur again.